The present disclosure relates to a flat cable serving as an interconnecting cable for various components disposed inside various electronic devices.
Related art electronic devices such as a personal computer, a flat-screen television set, a printer, and a scanner often employ a flat cable serving as an interconnecting cable for various components disposed therein. A flexible printed circuit board type of the flat cable is produced by an etching method. However, such a type of the flat cable is costly, and a length thereof cannot be greater than 1,000 mm due to a manufacturing infrastructure, causing difficulty in being applied to the increasing size of the flat-screen television set.
A flexible flat cable produced by a laminating method, on the other hand, has attracted attention as a substitute for the flexible printed circuit board type of the flat cable. The flexible flat cable has good flexibility and can be used in a pivotable portion. Moreover, the production cost and the unit price of the flexible flat cable are lower than those of the flexible printed circuit board type. Accordingly, the flexible flat cable tends to be applied to a wide variety of fields.
Conventionally, the flexible flat cable is not demanded to have an electrical characteristic such as a characteristic impedance. For example, a prior art flexible flat cable is provided by sandwiching a central conductor 101 from both sides by a base film 103 and laminating the base film 103 sandwiching the central conductor 101, so that both sides of the base films 103 are adhered as illustrated in FIG. 8. Such a prior art flexible flat cable is supposed to satisfy specifications needed. Herein, the base film 103 is, for example, made of polyethylene terephthalate, and includes a prescribed adhesion layer 102 applied thereto.
A flat cable of recent years, on the other hand, is demanded to increase the signal transmission speed with the development of various electronic devices such as a notebook personal computer and a flat-screen television set having high definition image quality. Moreover, the increase in the signal transmission speed is technically necessary for other electronic devices with the advance of digitization.
Such an increase in the signal transmission speed is in need of controlling the characteristic impedance of the cable. Accordingly, an impedance control cable in which the characteristic impedance is controlled is expected not only to enhance the capabilities thereof but also to be produced at a low price.
A flat type of the impedance control cable with a microstrip structure is illustrated in FIG. 9, and another flat type of the impedance control cable with a strip structure is illustrated in FIG. 10. In the impedance control cable with the microstrip structure, for example, a ground 203 is positioned on one surface of a transmission path formed of a conductor 201 and a dielectric substance 202 as illustrated in FIG. 9. In the impedance control cable with the strip structure, for example, a ground 303 is positioned on both surfaces of the transmission path formed of a conductor 301 and a dielectric substance 302 as illustrated in FIG. 10. The impedance control cables with the microstrip structure and the strip structure are already introduced in a market. Particularly, the impedance control cable with the microstrip structure is already employed in a certain flat-screen television set.
Such a flexible flat cable capable of controlling the characteristic impedance is illustrated in FIG. 11. A periphery of a central conductor 401 is covered with a dielectric substance 402, and a periphery of the dielectric substance 402 is covered with an outer conductor 403 as illustrated in FIG. 11, thereby forming the flexible flat cable capable of controlling the characteristic impedance. Such a flexible flat cable attracts attention as a substitute for an extra-fine coaxial cable of a high-end model from a low cost standpoint.
For example, Patent Document 1 discloses a flexible flat cable with a technology attempting to control the characteristic impedance thereof.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-31033
Particularly, Patent Document 1 discloses a flexible flat cable including: a conductor line formed of a plurality of conductors arranged in parallel; a foam insulator including an adhesive layer sandwiching the conductor line from both sides and then being laminated; and a metal layer including a conductive adhesive layer sandwiching the foam insulator including the adhesive layer from both sides. The conductor line is sandwiched by the foam insulator from both sides, and then the foam insulator sandwiching the conductor line is laminated, so that a dielectric constant of the foam insulator is combined with that of the air, allowing the dielectric constant to be lower than that of a related art insulator having no foam in a complex dielectric constant. Therefore, an electrostatic capacity of the character impedance is controlled, thereby setting the character impedance to be 50Ω. In such a flexible flat cable, a thickness of the foam insulator is relatively high, for example, between 150 μm and 250 μm, and an aluminum foil and a base film are laminated to serve as the metal layer including the conductive adhesive layer.
A signal transmission cable generally deteriorates, for example, noise resistance thereof as signal transmission speed increases. Accordingly, the signal transmission cable is demanded to be capable of handling the high-speed transmission. However, the increase in the signal transmission speed causes a problem of unnecessary radiation, or namely electromagnetic interface (EMI). That is, leakage of electromagnetic interface noise (radio wave) cannot be tolerated as the signal becomes a high frequency in the signal transmission. Such a leaked noise is provided to, for example, a cable adjacent to the signal transmission cable, causing adverse influence such as malfunctions and transmission losses.
The leakage of the noise can be reduced by sealing a noise source with a metal film. In this regard, the flexible flat cable with the microstrip structure including the ground 203 disposed on one surface of the transmission path is not expected to control the radiation with respect to a surface opposite to a surface on which the ground 203 is disposed as illustrated in FIG. 12. Consequently, the flexible flat cable with such a microstrip structure has a problem of controlling the radiation, causing a decrease in the likelihood of being employed in a case of being mounted in a product.
In the flexible flat cable with the strip structure as illustrated in FIG. 13, on the other hand, the ground 303 on the both surfaces serve as a shield layer, thereby being appropriate for controlling the radiation. However, the shield layer does not control an electrical characteristic. The ground 303 is positioned on the both surfaces of the transmission path, so that bonding strength of the transmission path and the ground 303 is increased, causing a problem of lowering the impedance. Consequently, such a type of the cable reduces the occurrences of lowering the impedance by methods for, for example, narrowing a width of the transmission path, lowering the dielectric constant of the dielectric substance, and widening space between the transmission path and the ground by an increase in a thickness of the dielectric substance.
Herein, since a feasible transmission path with or a feasible dielectric constant is limited, the method for widening the space between the transmission path and the ground is mainly employed among the methods to reduce the occurrence of lowering the impedance in the flexible flat cable with the strip structure. Accordingly, such a type of the flexible flat cable increases the thickness thereof, causing reduction of flexibility thereof. Consequently, the flexible flat cable has a problem of being not wired in a flexible manner inside an electronic device mounted. The flexible flat cable is preferably formed in thin from a standpoint of stress applied in the course of bending thereof. Such problems are attributed to the strip structure, and a number of manufacturing companies have attempted to solve such problems. However, a flexible flat cable satisfying the electric characteristics is not yet manufactured, and an increase in a cost by complication of a manufacturing method is currently concerned.